Profoundly impaired motor dysfunction is a major consequence of stroke. As a result, a large number of the more than 700,000 people in America sustaining a stroke each year have limitations in motor ability and compromised quality of life. Therapeutic interventions designed to enhance motor function and promote independent use of an impaired upper extremity following stroke are quite limited. There is a need to translate unique behavioral techniques shown to have impact on plasticity in the nervous system into practical, evidence-based, therapeutic interventions, especially at a time when the duration and number of treatments have been retracted. One such technique, derived from basic research with animals and human subjects has produced results that can substantially reduce the incapacitating deficits of many patients with chronic stroke and can increase their independence. The techniques, termed Constraint-Induced (CI) Movement Therapy or Forced Use, involve motor restriction of the less affected upper extremity for two weeks. Over this time, repetitive use of the more affected upper extremity is promoted for many hours a day. This treatment produces long-lasting improvements in real world extremity use among patients who are more than 1 year post-stroke and who have an ability to initiate some extension in wrist and digit joints. This intervention has never been studied across facilities systematically, nor has it been applied to sub-acute patients. Pilot work from 6 sites indicates that CI therapy may be as effective for sub-acute patients, 3-6 months post-stroke, as it is for more chronic patients. This prospective, 5-year, randomized national clinical trial, including 2-year follow-up will use a crossover design. Sub-acute patients with stroke with at least minimal ability to extend at wrist and digits (N=240) will be randomly assigned to receive CI therapy or the customary care normally available to them; 40 patients will be tested at each of 6 sites. One year after intake, control patients will be crossed over to receive CI therapy. All patients will be stratified into equal numbers of higher and lower functioning patients, categorized by movement criteria. Primary outcome measures are a laboratory motor function test and amount of extremity use in the real world setting. Changes in psychosocial functioning will also be measured.